Implantable electrical medical stimulation and/or sensing leads intended for chronic implantation in the body are well known in the fields of cardiac stimulation and monitoring, including cardiac pacing and cardioversion/defibrillation (C/D), and in other fields of electrical stimulation or monitoring of electrical signals or other physiologic parameters.
An implantable cardioverter/defibrillator (ICD) implantable pulse generator (IPG) or pacemaker IPG or an implantable monitor is typically coupled to the heart through one or more of such cardiac leads having a lead body extending between a proximal lead connector assembly and a distal stimulation and/or sense electrode or electrodes. The lead body typically comprises one or more insulated conductive wire extending through a lead body lumen of insulating outer sleeve formed of silicone rubber or polyurethane. Each conductive wire couples a proximal lead connector element with a distal stimulation and/or sensing electrode. A cardiac lead having a single stimulation and/or sensing electrode at the lead distal end coupled to a single conductive wire extending to a single proximal connector element is referred to as a unipolar lead. A cardiac lead having two or more stimulation and/or sensing electrodes at the lead distal end and two or more conductive wires each extending to a proximal connector element is referred to as a bipolar lead or a multi-polar lead, respectively.
In use, the distal end of an endocardial lead is transvenously advanced from a surgically created subcutaneous access into a vein through the venous route and into a heart chamber or cardiac blood vessel, and typically active or passive fixation mechanisms are employed to maintain the distal electrodes at a selected implantation site. The distal end of an epicardial lead is advanced through the pericardial space to a site of active or passive fixation with the epicardium or myocardium. One or more connector element of the proximal lead connector assembly of such an endocardial and epicardial lead is connected with a connector header of the IPG or monitor that is then implanted at a subcutaneous site of the patient's body.
The heart beats approximately 100,000 times per day or over 30 million times a year, and each beat stresses the lead conductors and lead body insulation. Over the years of implantation, the lead conductors and insulation are subjected to cumulative mechanical stresses as well as material reactions that can result in degradation of the insulation or fractures of the lead conductors with untoward effects on performance and patient well being. The lead bodies of pacing and ICD leads are subjected to continuous stretching and flexing as the heart contracts and relaxes and are formed to be highly flexible, resilient, and durable employing durable, bio-compatible, lead conductor and insulator materials and structures.
In light of these considerations, lead conductors of early bipolar and bipolar cardiac pacing leads were formed employing a stainless steel or MP35N alloy wound into a wire coil threaded through a lead body lumen. The wire coils helped resist stress and ensuing fracture, and it was also possible to insert a stiffening stylet into the wire coil lumen to stiffen the lead body during transvenous implantation of endocardial pacing leads.
Many current endocardial pacing leads employ multi-filar, parallel-wound, coiled wire conductors electrically connected in common in an electrically redundant fashion as a single polarity lead conductor in each of the unipolar, bipolar and multi-polar lead configurations. Such redundant coiled wire conductors of bipolar and multi-polar lead bodies are coaxially arranged about the stiffening stylet receiving lumen and insulated from one another by coaxially arranged insulating sheaths separating each coiled wire conductor from the adjacent coiled wire conductor(s). The number of separate lead conductors that can be incorporated in a lead body of a given diameter is limited in this coaxial winding approach.
Stiffening stylets were not typically used in implantation of epicardial pacing leads, and so stranded wire conductors, e.g., tinsel wire, were employed in the epicardial lead bodies of epicardial screw-in leads. In addition, highly flexible, silver core, drawn-brazed-strand (DBS) wire conductors were employed in early epicardial leads as described in U.S. Pat. No. 3,333,045. The '045 patent describes a silver core, DBS wire formed from a central silver filament surrounded by six silver-coated, stainless steel filaments that are drawn through a forming die while subjected to annealing heat to cause the silver coating to flow and braze the outer stainless steel and inner silver filaments together to form a DBS wire strand. Seven of the silver core DBS wire strands are then twisted around one another to form a 1×7 cable. Seven of the 1×7 cables are then wound and twisted together, preferably in a reverse spiral, to form a coiled, 7×7 strand, silver core, DBS wire cable (a 7×7 cable) or stranded wire conductor. Two of the 7×7 cables are loosely contained in lumens of a pair of silicone rubber tubes that are also filled with silicone fluid lubricant, and an outer silicone rubber sheath surrounds the pair of silicone rubber tubes.
This comparatively complex form of stranded wire conductor was not employed in clinically released unipolar and bipolar cardiac pacing leads for many years. However, more recently developed multi-polar cardiac leads, e.g., combined pacing and C/D leads, require an increased number of lead conductors in the lead body as well as high conductivity of at least the C/D lead conductors to more efficiently conduct C/D current. These needs have led to the development of lead bodies having multiple lead conductors including coiled wire conductors and substantially straight, high conductivity, stranded wire conductors that are electrically and mechanically coupled between a proximal connector element and to a distal electrode or terminal. Combinations of substantially straight, stranded wire conductors and at least one coiled wire conductor providing a stylet lumen are illustrated in commonly assigned U.S. Pat. Nos. 5,584,873, 6,052,625, and 6,285,910, for example. In these lead bodies, ETFE or PTFE sleeve insulators encase the stranded wire conductors, and a PTFE sleeve insulator surrounds the multi-filar, coiled wire, conductor. The insulated wire conductors are received in side-by-side lumens of an elongated lead body insulator that also incorporates elongated, empty, compression lumens.
The stranded wire conductors can take any of the forms disclosed in U.S. Pat. No. 5,760,341 and in the above-referenced '873 patent, for example. The number and composition of the individual strands of the stranded wire conductors vary as a function of the expected level of current to be carried and as a function of the material of which the wires or filaments are fabricated. High conductivity stranded wire cables are fabricated in a manner similar to that described in the above-referenced '045 patent employing silver core, MP35N wire filaments that are fabricated using a DBS fabrication process or a drawn filled tube (DFT) fabrication process well known in the art. In certain instances, the filaments are twisted into 1×7 strands, and then seven of the 1×7 strands are twisted about one another to form a substantially straight, 7×7 strand cable. In other instances nineteen of the filaments are twisted about one another to form a 1×19 cable. The 1×19 and 7×7 cables are then typically coated with an insulating layer to protect the cable strands and facilitate handling and provided for fabrication of cardiac leads.
During fabrication of a cardiac lead, it is necessary to make an electrical and mechanical connection of the 7×7 cable or 1×19 cable proximal end and distal end with a lead connector element and an electrode, respectively. It is necessary to cut the 7×7 cable or 1×19 cable into a cable length sufficient to extend through a lead body lumen and to make the electrical and mechanical connections and to trim away the insulation or otherwise expose a cable connection end section. The electrical and mechanical connections of the lead connector element and electrode to the exposed proximal and distal cable connection end sections are made employing mechanical crimping and/or laser welding techniques.
The exposed proximal and distal cable connection end sections can be damaged or can be frayed in the course of cutting or trimming the insulation from the 7×7 cable or 1×19 cable proximal end and distal end. The twisted strands or filaments can unravel and spread apart in the manner shown in the above-referenced '045 patent. It is suggested in the '045 patent that the cut ends of the 7×7 cable be “tinned” to prevent fraying during winding of the 7×7 cable into a helix. But, tinning with low melting temperature metals, other than perhaps molten silver, would introduce compounds or elements that would not necessarily be bio-compatible and that could degrade the insulating materials or make the “tinned” ends difficult to mechanically and electrically attach to a connector element or electrode.
To avoid such problems, tinning is not used, and it has become standard practice to laser weld the 7×7 cable or 1×19 cable proximal and distal ends either in conjunction with the attachment to the lead connector element and electrode, respectively, or in a separate step. Butt welding techniques for laser welding tinned or un-tinned stranded wire ends to electrode pins are described, for example, in U.S. Pat. No. 5,269,056.
Laser welding the 7×7 cable or 1×19 cable proximal and distal ends to prevent fraying requires relatively bulky, expensive, precision laser welding equipment as can be seen from the three laser beam equipment disclosed in the above-referenced '056 patent that increases costs of fabrication.
Consequently, a need remains for low cost methods and equipment for adhering the filaments or strands of the cut ends of such stranded wire conductors together during fabrication of electrical medical leads that does not introduce impurities and provides consistent results.